The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Polysaccharides and cellulosic polymers or their derivatives are typically used to provide viscosity in these fluids. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications.
Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions, where rates of cross-linking with known cross-linking compositions may be unacceptable.
U.S. Pat. No. 4,477,360 discloses a cross-linking composition comprising a zirconium salt or chelate and a polyhydroxyl-containing compound having from about 3 to about 7 carbon atoms, and optionally an aqueous fluid or an alkanol, for use in fracturing a subterranean formation. While this composition provides a rate of cross-linking that is often desirable in hotter, deeper wells, it provides insufficient initial viscosity development to carry sand into the formation to the extent desired.
U.S. Pat. No. 4,883,605 discloses a water-soluble zirconium chelate formed from a tetraalkyl zirconate and hydroxyethyl-tris-(2-hydroxypropyl)ethylenediamine, and the use of the chelate as a cross-linking agent in hydraulic fracturing fluids and in gels that are used for selectively plugging permeable zones in subterranean formations or for plugging subterranean leaks. Co-pending U.S. patent application Ser. No. 11/643,513, filed Dec. 21, 2006, discloses a related complex having a 1:1 molar ratio of zirconium and N,N,N′,N′-tetrakis-(2-hydroxypropyl)-ethylenediamine.
The products of U.S. Pat. No. 4,883,605 and U.S. patent application Ser. No. 11/643,5513 may be used as cross-linkers for use in many hotter, deeper oil well applications. However, at high pH conditions (such as pH 10), where polysaccharides are most stable, the products of U.S. Pat. No. 4,883,605 cross-link too slowly (>10 minutes), causing a “sand out” to occur, which is the result of sand depositing at the bottom of the wellbore due to lack of viscosity development before the gel reaches the fracture zone. The products of co-pending U.S. patent application Ser. No. 11/643,513 cross-link in the desirable range, which is 3-8 minutes, as illustrated by testing in a FANN viscometer at 275° F. (135° C.) and 122 rpm at 212 reciprocal second of shear. (The FANN results provide a means to predict performance in oil well operation.) Although the products of co-pending U.S. patent application Ser. No. 11/643,513 can be used in many hotter, deeper wells, they do not generate as high a viscosity as desired to maintain the sand in suspension for the length of time needed in hotter, deeper wells having high pH.
Commercially available zirconate cross-linkers, such as tetra-triethanolamine zirconate cross-link too fast under high pH conditions, causing a significant loss in viscosity due to shear degradation, which can also result in a sand out. Nonetheless, it is desirable to use a cross-linking composition at pH 10 or higher, where polysaccharides used in cross-linking compositions are most stable.
There is a need for compositions which cross-link at a rate intermediate between zirconium complexes of hydroxyethyl-tris-(2-hydroxypropyl)ethylenediamine and triethanolamine zirconates, and generate and maintain sufficient viscosity to be used successfully at high pH (about pH 10 and above) and high temperature (325°-400° F., 163-204° C.) conditions.